1. Field of the Invention
The present invention relates generally to a three-phase brushless motor and, more particularly, to a three-phase brushless motor particularly adapted for audio and video equipment.
2. Description of the Background
There have been proposed heretofore flat, miniaturized three-phase brushless motors in which the stator coils are distributed uniformly about a stator plate and in which the coils are connected either in series or parallel. Some sort of electromagnetic transducer is employed to sense the relative rotational position of the magnetic poles on the rotor and produce appropriate switching signals so that the proper currents can flow through the coils arranged on the stator. One kind of heretofore proposed brushless three-phase motor involves the use of three such electromagnetic transducers, however, this causes a problem when it is desired to miniaturize the motor because the positional accuracy of the transducers is critical and, also, the transducers occupy more than a nominal physical space on the stator plate.
To overcome this drawback, a brushless three-phase motor has been proposed in which only two electromagnetic transducers are employed, with a resistive summing network used to provide a third input for the three phases of the stator windings. In that situation, a different problem arises in that the waveforms of the electromagnetic transducer elements are flattened and saturated so that the slope of the summed or composite signal at the zero crossing is made gradual, thereby leading to an increase in rotational torque ripple of the motor.
In copending U.S. patent application Ser. No. 654,004 filed Sept. 12, 1984 and assigned to the assignee hereof, of which this present application is a continuation in part, portions of oppositely poled elements are arranged on the main magnetic poles in an effort to increase the slope of the electromagnetic transducer output signals at the zero crossing points and thereby decrease or suppress rotational torque ripple. Nevertheless, the ripple is not suppressed to an optimum extent, and the previously proposed approaches all require high accuracy and correlation between the several electromagnetic transducers employed.